Problem: logging of utf8-incompatible binary strings didn't work
Fix: hex-encoding of incompatible sequences.


mysql-test/r/log_tables.result:
  Adding test
mysql-test/t/log_tables.test:
  Adding test
sql/field.cc:
  Copying with hex escaping
sql/field.h:
  New field flag
sql/log.cc:
  Marking the column "general_log.argument" as hex-escaping field.
sql/sql_string.cc:
  New function to copy strings with hex-encoding of incompatible characters.
sql/sql_string.h:
  Prototype for the new function